Amorphous alloys exhibit a number of differences in their properties from the normal crystalline form of the same alloys which make them especially suitable for certain applications. Amorphous alloys are harder, more abrasive and more sensitive to mechanical stresses and have higher mechanical strength, flexibility and electrical resistivity than the crystalline forms of the same alloys. Some amorphous alloys exhibit the softest magnetic characteristics of any known materials. This latter property is especially desirable for magnetic core materials since the ease with which the material can be magnetized and demagnetized controls the hysteresis losses experienced. This becomes important where the magnetic material is repetitively magnetized in opposite directions as is, for example, the case in magnetic cores of the type used in AC machinery.
Because of the way amorphous alloys are formed, only thin ribbons can be produced and such ribbons have a maximum thickness of about 0.076 mm. Typical thicknesses are from about 0.015 mm to about 0.05 mm. By contrast, transformer core laminations are normally about ten times thicker. Thus, about ten times as many layers of amorphous metal ribbons are required to form a transformer core structure of a given cross section as are required to form the same structures using prior art transformer materials.
An important consideration in many applications where magnetic cores are used is the space factor. The space factor may be defined as the ratio of the volume of core material within the built up core to the volume of the built up core itself. The space factor is important because if the layers making up the core do not lie flat upon each other but remain separated by air or other non-magnetic material, the volume of the core will increase without a corresponding increase in its desirable magnetic properties. Ribbon irregularities decrease the space factor. Thus, if burrs or irregularities are present on the edges of the core laminations, the laminations will not lie flat and consequently the space factor is degraded. The thinness dictated by the way amorphous metals are made adds to the problem. For example, edge or surface irregularities which are small enough to be ignored in conventional core laminations, may cause severe degradation of the space factor when ten or more times as many layers are used.
The deformation of any material requires the material to flow as it is forced or worked. At low temperatures the flow of amorphous alloys is governed by an inhomogeneous deformation mechanism characterized by high stress. The high stress causes two problems: a high rate of tool wear and loss of magnetic properties. If subjected to high stress, the tools used in the forming operations of articles that consist of such alloys will have only short useful lives. It is also known that inhomogeneous deformation of amorphous alloys can be detrimental to the soft magnetic properties of the alloys. It is therefore desirable to avoid such high stress in deforming or working amorphous alloys.
As disclosed in Japanese patent application No. 32288 to T. Masumoto, published Nov. 5, 1976, some of the difficulties in forming amorphous alloys can be overcome or reduced by performing the forming operations at elevated temperatures. As set forth in that publication, forming processes should be applied to the amorphous alloy only at temperatures above the "ductile transition temperature", herein designated T.sub.p. This same temperature, which is regarded as critical for working amorphous alloys, is also referred to as the "plastic transition temperature" in an article by Liebermann, in Mat. Sci. Eng. Vol. 46, Pg. 241 (1980). It is known that amorphous alloys can be deformed above this plastic transition temperature at low stresses to a high degree of straining. Such hot forming of a metallic glass at low stress is reported in D.H.R. "Proceedings Third International Conference on Rapidly Quenched Metals", by J. Patterson, A. L. Greer, J. A. Leake (Chameleon Press, 1978), pg. 293 and was demonstrated by drawing a cup from a ribbon of amorphous alloy. As reported in Scripta Met. Vol. 14, Pg. 1331 (1980), strains approaching 100% in an amorphous alloy ribbon of PdFeSi were produced at stresses as low as 150 Mpa when deformation of the ribbon was carried out at high temperatures.
In none of the foregoing studies, nor in the methods developed from the studies, was any concern given to the effect of the rate of heating on the forming of the amorphous article. Primary consideration in each prior instance was given to the crystallization kinetics of the alloy. The object in these prior efforts was to effect the working of the alloy without imparting significant degrees of crystallinity to the article being formed and to retain the amorphous character of the alloy. The avoidance of crystallization was recognized in these prior efforts as a primary consideration in preserving the properties of the amorphous alloys.
U.S. Pat. No. 4,584,036, assigned to the present assignee, discloses a relationship between the softening and increase in workability of an amorphous alloy article and the rate at which the article is undergoing heating. As set forth in that patent, the heating history of the article, that is the heating or rate of heating to a certain temperature prior to working, must be distinguished from the effective rate at which an article is being heated at the time the working or forming of the article is taking place. This patent further discloses that amorphous alloys undergo a softening during the time when they are being heated at a relatively high heating rate. Further, the variation of the softening temperature with, or as a function of, the heating rate was determined in a quantitative manner.
U.S. Pat. No. 4,715,906, which is likewise assigned to the assignee of the present application, discloses a slightly different heating regimen. According to this regimen for heating rates of 1000.degree. C./min. or higher, the viscosity of the alloy is so low that the softening window is enlarged in a temporal sense. The softening window is the difference between the temperature at which the alloy softens and that at which it crystallizes. When the heating rate is high enough this window is large enough for the amorphous alloy to retain its ability to be worked in an apparent "soft" state, even though the alloy is experiencing an isothermal hold of one to several seconds. Generally, the higher the rate of heating above 1000.degree. C./min., the longer the isothermal hold which can be tolerated by the amorphous alloy article without loss of its favorable magnetic properties. These findings and the quantitative relationships developed that define the isothermal window, are all set out in the last-mentioned patent.
U.S. Pat. No. 4,670,636, assigned to the present assignee, discloses how the softening technique can be utilized to provide a method for parting a bundle of amorphous alloy articles. This is done by applying a tensile force to the article while rapidly heating a seam of the top article to be cut. The top article separates into parts and the separated parts of the top article are then withdrawn and the next article is exposed to the applied heat. It is known that heating of an amorphous alloy strip in a narrow region causes buckling and distortion of the strip, even though the heating is not sufficient to melt or even soften the material of the narrow region. Therefore, heating under the influence of tension is helpful in minimizing this property to keep the ribbon flat.
In comparison to their properties before heating amorphous alloys become more brittle upon subsequent cooling. Therefore, heating of amorphous alloy ribbons causes undesirable embrittlement of the ribbons that can lead to cracking and chipping of the ribbons when they are stressed. Additionally, amorphous alloy ribbons separated under tension in a heated region, neck in the heated region. As a result, excessive necking in the heated region can cause an undesirable reduction in the thickness and width of the ribbon at the separated edge.
Accordingly, it is one object of this invention to provide a method which permits a deck of amorphous alloy ribbons to be separated without the use of cutting tools in one operation.
Another object is to avoid and prevent the formation of meltbeads, burrs, or any substantial cracking or chipping at an edge formed by separation, thereby improving the stackability of the severed edges of cut ribbon.
Another object is to provide a method for separating a deck of amorphous alloy ribbons without significantly detracting from the beneficial magnetic properties of the alloy.
Another object is to provide a method for separating a deck of amorphous alloy ribbons while minimizing the portion in the ribbons that is heated and necked.